This invention relates generally to a combination manifold pressure and ambient pressure sensor and, more particularly, to a manifold absolute pressure and ambient pressure sensor which is utilized in conjunction with a control system for a fuel injection system to provide altitude compensation, fuel control, ignition control or exhaust gas recirculation control.
While the sensor and the system of the present invention will be described in conjunction with the provision of altitude compensation of the fuel control of an electronic fuel injection system, it is to be understood that other uses of the various pressure signals may be provided, as for example fuel trim, ignition or spark advance control, or exhaust gas recirculation control. Provision has been made in the past for altitude compensation of a fuel injection system. Normally, the calibration of the control unit for the control of the fuel being fed into the engine is made at sea level. However, with engine operation taking place in a range at or below sea level to high altitudes, it is necessary to compensate the fuel delivery in response to the operation of the vehicle at altitude. If this compensation is not provided, the engine typically will operate on the cruise portion of the fuel law and the operator will be unable to accelerate the vehicle except by accelerating in the wide open throttle mode of operation.
In prior systems, altitude compensation has been provided by deriving an ambient absolute pressure signal through the use of a barometric pressure sensor to provide the necessary ambient pressure signal for the fuel control unit. As can be appreciated, the system can be costly in that it requires an additional sensor and, through the additional component, increases the possibility of failure of the system.
Other schemes have been provided, as for example as disclosed in the Todd L. Rachel U.S. Pat. No. 3,931,808, issued Jan. 13, 1976, wherein the manifold absolute pressure sensor is utilized for a dual purpose, i.e., to provide a constant manifold absolute pressure signal for use by the electronic control unit in controlling the fuel and, periodically, providing a barometric pressure signal in response to certain engine operation conditions. In the specific implementation described in the above referenced Rachel patent the manifold absolute pressure sensor is actuated during cranking of the engine to provide a barometric pressure signal. This signal is utilized to adjust the electronic control unit in accordance with the barometric pressure sensed during cranking. Subsequently, during wide open throttle operation, the manifold absolute pressure signal is sensed to update the barometric pressure signal due to the fact that the manifold absolute pressure sensor signal output is very nearly barometric pressure at wide open throttle operation. However, with this prior system, the stored information with respect to barometric pressure is updated only when the engine is operated in the wide open throttle mode of operation, which may be infrequently.
With the system of the present invention, the information stored with respect to the ambient absolute pressure is updated on a high frequency basis and particularly when the engine manifold absolute pressure, with respect to ambient pressure, exceeds a preselected amount. The preselected amount is preset into the combination sensor during the manufacturing process.
The basic theory of operation of the sensor of the present invention is best understood when it is appreciated that the absolute ambient pressure may be derived from a pair of signals which are generated by the combination sensor of the present invention. The combination sensor generates a first signal which is indicative of a preset relationship between the ambient pressure and the manifold absolute pressure. When this condition occurs, a switch is operated to enable a sample-and-hold circuit to sense the manifold pressure at the time the switch is actuated. With this information, the sensed manifold pressure may be added to the preset differential pressure sensed between the ambient pressure and the manifold pressure to provide an absolute ambient pressure signal. If this ambient absolute signal is further processed by subtracting the sensed manifold pressure therefrom, an engine vacuum signal may be generated which is referenced to ambient pressure. This latter signal may be utilized to change the basic calibration for ignition or exhaust gas recirculation control on a step basis rather than on a comtinuous basis.
Accordingly, it is one object of the present invention to provide an improved and ambient absolute pressure signal sensor.
It is another object of the present invention to provide an improved manifold vacuum pressure signal sensor.
It is still another object of the present invention to provide an improved combination manifold absolute pressure and ambient absolute pressure sensor.
It is still a further object of the present invention to provide an improved sensor of the type described which is inexpensive to manufacture, easily installed and reliable in operation.